1. Field of the Invention
The present invention generally relates to a semiconductor device and, more particularly, to a structure of a multilayer wiring substrate provided with differential signal wires, and a semiconductor device having such a multilayer wiring substrate and a semiconductor element mounted thereon.
In a multilayer wiring substrate, a signal layer having a signal wire is provided between a power-supply layer and a ground layer so as to match an impedance of the signal wire to reduce a crosstalk noise. A structure having the power-supply layer and the ground layer placed respectively on both sides of the signal layer is referred to as a stripline structure. A structure having the power-supply layer and the ground layer placed on one side of the signal layer is referred to as a microstripline.
2. Description of the Related Art
Japanese Laid-Open Patent Application No. 05-136567 discloses a structure in which a power-supply portion and a ground portion are placed in a same layer. In a multilayer wiring substrate disclosed in this Japanese Laid-Open Patent Application No. 05-136567, each of a power-supply wire and a ground wire is formed in a comb-like form, and tooth portions of the power-supply wire and the ground wire are interlocked one another with a predetermined distance in between so as to form a power-supply layer.
Japanese Laid-Open Patent Application No. 09-18156 discloses a structure in which power-supply wires are formed in a mesh-like form in the whole substrate. Specifically, a multilayer wiring substrate disclosed in this Japanese Laid-Open Patent Application No. 09-18156 comprises: a first layer having a first signal wiring portion, a first power-supply wiring portion, and a plurality of first ground wiring portions; and a second layer having a second signal wiring portion, a second power-supply wiring portion, and a plurality of second ground wiring portions. The second ground wiring portions of the second layer are connected to the first ground wiring portions of the first layer so that ground wires are formed in a mesh-like form in the whole multilayer wiring substrate, reducing an inductance and an impedance of the whole ground wiring portions.
As described above, by devising a structure of a power-supply portion and a ground portion of a multilayer wiring substrate, electric properties of the multilayer wiring substrate are improved. However, in a case where a wiring density of the multilayer wiring substrate is increased, and the multilayer wiring substrate is required to operate at a high-frequency band, it is difficult to match an impedance of a signal wiring portion. This makes it difficult to restrain a crosstalk noise between signal wires. Especially in a case where differential signal wires and a general signal wire are present together in a same layer, it is further difficult to match the impedance of the signal wiring portion.
Therefore, since the general signal wire and the differential signal wires provide greatly different characteristic impedances in the signal wiring portion, a portion provided with the general signal wire and a portion provided with the differential signal wires need to have different characteristic impedances. Generally, the characteristic impedance of the signal wiring portion can be changed by varying a thickness or a dielectric constant of an insulating layer in which the signal wiring portion is provided. Also, the characteristic impedance of the signal wiring portion can be changed by varying a width or a thickness of a signal wire.
At this point, although it is possible to change the over-all thickness of an insulating layer, it is difficult to change the thickness of an insulating layer partially. Additionally, it is also difficult to use different materials in one insulating layer from a practical point of view. Therefore, a width or a thickness of a wire needs to be changed so as to form a wiring layer in which wires having different impedance characteristics are present together. However, there is a disadvantage that, with wires becoming finer and finer, it is still difficult to sufficiently adjust impedance characteristics only by changing a width or a thickness of a wire.
It is a general object of the present invention to provide an improved and useful multilayer wiring substrate and a semiconductor device having the multilayer wiring substrate in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide a multilayer wiring substrate having a power-supply portion and a ground portion arranged effectively in each layer of the multilayer wiring substrate so as to provide optimal impedance characteristics for both of a general signal wire and differential signal wires, and a semiconductor device having the multilayer wiring substrate.
In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention a multilayer wiring substrate comprising:
differential signal wires placed within a first insulating layer between a first power-supply plane and a first ground plane; and
general signal wires placed within a second insulating layer between a second power-supply plane and a second ground plane,
wherein the differential signal wires are placed in a different plane from a plane having each of the general signal wires so that the different plane includes a first area having the differential signal wires, and a second area having one of the second power-supply plane and the second ground plane; and the general signal wires are placed in a vertical direction of the second area in a laminated state so that each of the general signal wires is placed between the second power-supply plane and the second ground plane.
According to the present invention, since the general signal wire and the differential signal wires are placed in different planes, the distance from the general signal wire to the ground plane or the power-supply plane and the distance from the differential signal wires to the ground plane or the power-supply plane can be made different. Since an impedance regarding a signal wire depends on a distance from the signal wire to a ground plane and a distance from the signal wire to a power-supply plane, the impedance regarding the general signal wire and the impedance regarding the differential signal wires can be made different according to the distance from the general signal wire to the ground plane or the power-supply plane and the distance from the differential signal wires to the ground plane or the power-supply plane. Therefore, a multi-layer wiring substrate having both of a general signal wire and differential signal wires with a simple structure can be easily manufactured.
Additionally, the multilayer wiring substrate according to the present invention may further comprise a first plane having only the first ground plane, and a second plane having the first power-supply plane formed in a vertical direction of the first area, the second plane having the second ground plane formed in the vertical direction of the second area,
wherein the differential signal wires are placed between the first ground plane and the first power-supply plane.
Additionally, the multilayer wiring substrate according to the present invention may further comprise a first plane having only the first power-supply plane, and a second plane having the first ground plane formed in a vertical direction of the first area, the second plane having the second power-supply plane formed in the vertical direction of the second area,
wherein the differential signal wires are placed between the first power-supply plane and the first ground plane.
According to the present invention, the differential signal wires are placed between the first power-supply plane and the first ground plane placed in the vertical direction of the first area. On the other hand, the general signal wires are placed in the vertical direction of the second area in a laminated state so that each of the general signal wires is placed between the second power-supply plane and the second ground plane. In the plane having the differential signal wires in the first area, the second power-supply plane or the second ground plane is formed in the second area thereof. Therefore, larger numbers of the power-supply planes and the ground planes can be provided in the vertical direction of the second area having the general signal wires placed in a laminated state. Therefore, the distance from the differential signal wires to the ground plane or the power-supply plane can be made larger than the distance from the general signal wire to the ground plane or the power-supply plane.
Additionally, in the multilayer wiring substrate according to the present invention, each of the differential signal wires, the general signal wires, the first power-supply plane, the first ground plane, the second power-supply plane and the second ground plane may be made of a predetermined conductive material, and each of the first insulating layer and the second insulating layer may be made of a predetermined insulating material.
Additionally, in the multilayer wiring substrate according to the present invention, the predetermined conductive material and the predetermined insulating material may be selected so that an impedance regarding the differential signal wires becomes a predetermined value, and an impedance regarding each of the general signal wires becomes a predetermined value.
According to the present invention, the differential signal wires, the general signal wires, the power-supply planes, the ground planes and the first insulating layers can be made of appropriate materials so that impedances regarding the differential signal wires and the general signal wires, which impedances depend on properties of these materials, can be adjusted.
In order to achieve the above-mentioned objects, there is also provided according to another aspect of the present invention a semiconductor device comprising:
a multilayer wiring substrate having: differential signal wires placed within a first insulating layer between a first power-supply plane and a first ground plane; and general signal wires placed within a second insulating layer between a second power-supply plane and a second ground plane, wherein the differential signal wires are placed in a different plane from a plane having each of the general signal wires so that the different plane includes a first area having the differential signal wires, and a second area having one of the second power-supply plane and the second ground plane; and the general signal wires are placed in a vertical direction of the second area in a laminated state so that each of the general signal wires is placed between the second power-supply plane and the second ground plane; and
a semiconductor element mounted on the multilayer wiring substrate.
According to the present invention, since the general signal wire and the differential signal wires are placed in different planes, the distance from the general signal wire to the ground plane or the power-supply plane and the distance from the differential signal wires to the ground plane or the power-supply plane can be made different. Since an impedance regarding a signal wire depends on a distance from the signal wire to a ground plane and a distance from the signal wire to a power-supply plane, the impedance regarding the general signal wire and the impedance regarding the differential signal wires can be made different according to the distance from the general signal wire to the ground plane or the power-supply plane and the distance from the differential signal wires to the ground plane or the power-supply plane. Therefore, a multi-layer wiring substrate having both of a general signal wire and differential signal wires with a simple structure can be easily manufactured. Additionally, with the multi-layer wiring substrate, a semiconductor device requiring a fine wiring structure can be easily manufactured.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.